Contemplate your own teeth for a second. We usually only think about them when there's something wrong -- a chip, a cavity, discoloration. But teeth themselves are incredibly durable, able to survive fire and outlast the rest of your body after death. Teeth are hard, which makes them useful for their main purpose in humans: chewing food. On the outside of the tooth, there's enamel, with hard materials called dentin and cementum below that. At the very center of the tooth is the pulp, where the blood and nerves are. (You might realize you have a cavity once the pulp is infected and hurts.) The hard outer layers protect the sensitive inner layers of the tooth.
A narwhal tusk is the exact opposite. The soft, sensitive part is on the outside, while the dense, hard part makes up the middle. Ten million tiny holes lie right on the surface on the tusk. Human teeth have these little tubules too, which is why sometimes the cold bothers your teeth, but they're covered with enamel. Imagine having all your nerves exposed in the icy waters of the Arctic. Why would the most sensitive part of a tooth be on the outside?
Dr. Martin Nweeia, a clinical instructor at the Harvard School of Dental Medicine, has one theory: The narwhal tusk acts like a sensor. Having all those nerves on the outside allows the whale to detect water pressure, temperature and salinity. It might even be able to detect barometric pressure when it's above the water's surface. However, if the tusk is so important to survival, why don't females have one? We don't know.
This sensitive quality does seem to cancel out some of the ruling theories about the tusk, such as the idea that male narwhals use them to duel, although some scientists still argue that the tusk is a way of establishing dominance. With nerves on the outside, it would seem such a battle would be painful for the whale. Narwhals do touch tusks, but observers have reported that it's not in a violent way. Researchers aren't sure why but suggest that perhaps it's a way of communicating. In the past, people also suggested that the tusk was used for breaking through ice or spearing prey, but no such evidence exists.
One theory still holds, which is that the tusk has something to do with mating, perhaps acting as a flashy sex characteristic. Almost all male narwhals have a tusk, but only about 15 percent of female narwhals do. There's still so much we don't know, and the narwhal isn't the easiest animal to study.
The narwhal tooth is the only straight tusk in the world -- all other known tusks are curved. It's also one of the only spiral teeth. The left tooth comes up through the jaw and corkscrews through the lip. Occasionally, someone spots a double-tusked narwhal, which happens when the whale's right tooth grows into a tusk too. But disdaining nature's liking for symmetry, this second tusk doesn't mirror the first -- it spirals in the exact same way as the other one, to the left. The tusk is flexible, able to bend about a foot in any direction without breaking. The tusk can grow up to 9 feet (2.7 meters) or more, which is amazing when you consider that the male is only about 15 feet (4.6 m) long at maturity.